Insulin receptors were identified on glomerular endothelial cells. Insulin binding was time, temperature and pH dependent and total specific binding was 2.5 +/- 0.3%/10(sixth power) cells. Binding analysis according to Scatchard resulted in a curvilinear plot. The kd of the high affinity site was estimated to be lxl0(tenth power) Down-regulation of the insulin receptor was time and concentration dependent. The receptor, visualized with SDS-PAGE and autoradiography following crosslinking, contained an alpha subunit Of Mr of 125,000 daltons, similar to that of other tissues. occupation of the receptor by insulin stimulated phosphorylation of the tyrosine kinase on the beta subunit, Mr 95,000 daltons, of the insulin receptor. Insulin stimulated glucose and amino acid uptake and was able to act as a weak progression factor. Therefore it was concluded that insulin is necessary for the metabolic function of glomerular endothelial cells, but not essential for DNA synthesis. It is conceivable that the abnormalities of the glomerulus in diabetes could be due to a dysfunctional endothelium, not able to perform its normal role in the glomerulus when presented with abnormal amounts of insulin.